Patent GB-689735-A (GB)
Title: Improvements in automatic telephone-exchange systems
Inventor: JOHNSON BENJAMIN BERTIE
IPC: H01H67/26
Filed: 1949-05-02
Abstract: 689,735. Automatic exchange systems. TELEPHONE MANUFACTURING CO., Ltd. May 1, 1950 [May 2, 1949], No. 11722/49. Class 40 (iv). A selector switch may be used as a group selector to gain access to a free outlet in a wanted level or as a final selector. The device uses " bar-relay switches " in which a bank of contacts is operated by an armature only when the finger magnet corresponding to that bank has been energized to interpose a finger between the armature and the bank of contacts. Group selector, Fig. 4. The group selector is associated with a pre-selector described in Specification 686,751 and may be approached over loop -a, +a or - b, +b depending on whether SW is operated or not. When the group selector is seized by operation of T, A pulls up and energizes B which releases T, earths the counting chain SR1-SR10 and energizes C which returns dialling tone to the subscriber. Line relay A repeats dialled impulses to the SR relays, the first impulse energizing SR1 which locks up, FP pulling up at the end of the impulse. Subsequent impulses energize successive relays SR2 and so on, the preceding relay falling back at the end of each impulse. The sixth impulse additionally energizes CH to distinguish between the two levels of banks in the selective relay unit, referred to later. Operation of each SR relay energizes a group of ten finger magnets, each group being represented by one of the blocks M-Q. Each operated finger magnet, one in each selective relay unit SRC, interposes a finger between the armature of relay R and the related contact bank m-q. When the last pulse of the train is received, C falls back slowly to release FP and operate R and RA. R then closes the selected contacts in each of the selective relay units SRC and releases the finger magnets M-Q. RA releases the operated SR relay, whereupon C re-energizes to re-operate SR1. Relay H then tests the selected P wire in the group P11-P51 or P61-P01, operating if the line is free. If the line is busy, SR2 pulls up to release SR1, both relays being, however, energized together momentarily but rectifiers MRB, MRA preventing any short-circuit between the related P wires if the P wire in the group P12-P52 or P62-P02 is free. The operation proceeds until a free line is found, when battery is returned over the P wire to operate H, which switches the incoming line forward and releases A. HA releases T and holds the operated SR relay. Relapse of A restores all relays except H, HA, CH (if operated) and the operated SR relay. Before B falls back, however, the A relay in the next stage operates to return earth over the P wire. If all outlets are busy, H cannot operate and busy tone and busy flash are returned by BT, BF over sr10/3, sr10/4. Final selector. When arranged as a final selector, the apparatus responds to a first train of impulses in the manner already described to select a bank of contacts in each of the selective relay units SRC. When, however, C reoperates, it energizes a relay G, Fig. 5, which energizes E. The second train of impulses then operates the SR relays again, after which C releases to connect H to the P wire of the wanted line and if the line is free, H energizes, locks up over its x contacts h1 and operates HA. Contacts h2 earth the P wire of the wanted line, h3 holds the operated SR relay and releases E, R, G, while h4, h5 complete a ringing circuit through F. Connection to lines controlled by the evennumbered SR relays is effected over contacts ha1-ha4, ha5 returns ringing tone to the calling subscriber and ha6 releases FP. When the called subscriber answers, F operates to energize D which reverses the current. If the called line is engaged, H cannot operate. C then releases G slowly to connect up busy flash and busy tone and disconnect H from the P wire. P.B.X. For calls to a P.B.X. group, a further set of contacts is provided, as shown, all contacts in the group being earthed except the last. If now, a called P.B.X. line is engaged, relay G is held by earth on the called line, which also operates the next SR relay and so on. The last line acts as a normal line. The Provisional Specification refers to Specification 689,736.